Oxidative Dissolution Of Pyrite Research Articles

Alumina inhibits pyrite oxidative dissolution by regulating solid film passivation layer and S, Fe, and Al speciation transformation

The oxidation of pyrite results in the formation of a solid film passivation layer on its surface. This layer effectively hinders the direct interaction between H2O, O2, and the pyrite surface, thereby impeding the oxidation dissolution of pyrite. There are few studies on whether alumina (Al2O3), a common aluminum-containing oxide, affects the formation of a solid film passivation layer on the surface of pyrite and inhibits the oxidation dissolution of pyrite. This research investigates the impact of Al2O3 incorporation on the speciation transformation of S, Fe, and Al on the surface of pyrite during oxygen pyrite process. The oxidation of pyrite followed the “polysulfide-thiosulfate” complex oxidation pathway. When <1.5 g/L Al2O3 was introduced, it increase pyrite oxidation, whereas ≥1.5 g/L Al2O3 prevented pyrite oxidation. The process of Al2O3 dissolution results in the consumption of H+ and the subsequent release of Al3+. This, in turn, facilitates the hydrolysis of Fe3+ and Al3+ to generate a secondary mineral layer on the pyrite surface. As a result of the accumulation of S promotes the formation of polysulfide chemical (FeSn) or iron deficiency sulfide (Fe1-xS), resulting in the formation of a solid film passivation layer composed of sulfur film and secondary mineral layer. The results demonstrated that Al2O3 can promote the formation of a solid film passivation layer on the surface of pyrite, which has significant implications for controlling the oxidation dissolution process of pyrite and offers a new perspective for the source control of acid mine drainage.

Chemical stability of Hg-adsorbed pyrite under different pH and redox conditions: A fundamental study for stable management of Hg-bearing wastes

Along with the worldwide awareness of mercury (Hg) toxicity, safe disposal and preservation of Hg-bearing waste becomes a matter of concern. In this study, Hg adsorbed pyrite, which is a type of Hg-bearing wastes, was synthesized and subjected to leaching tests at different pH (2, 6, and 10) and dissolved oxygen concentrations (aerobic and anaerobic) for 168 h in order to evaluate the chemical stability of the adsorbed Hg. Hg solubilization after 168 h leaching was negligibly small (0.13 µmol/L) at pH 10 under aerobic conditions while robust sulfur solubilization here (29.4 mmol/L), resulting from oxidative dissolution of pyrite, was expected to trigger the Hg desorption. Under anaerobic conditions, 35 times more Hg was released at pH 10 than pH 2, which was the opposite trend from aerobic conditions. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) analyses found that Fe (oxy)hydroxides were formed on the pyrite surface through its oxidative dissolution, contributing to the re-immobilization of once-desorbed Hg via the formation of surface complexation. Little Fe (oxy)hydroxide formed under anaerobic conditions, thus leading to the enhanced Hg release at alkaline than acidic pH values. Numerical calculation with a geochemical modeling confirmed that this hypothesis was valid and allowed the Hg distribution during the leaching of Hg-bearing pyrite to be quantified. Overall, we developed some suggestions for the stable management of Hg-bearing waste at the final waste disposal site based on the findings of this study.

Complex Speciation and Distribution of Iron, Sulfur, and Trace Metals in Coal Mine Soils Reflect Grain- and Sub-Grain-Scale Heterogeneity during Pyrite Oxidative Dissolution

Historical coal mining practices have caused various soil and water hazards, particularly through the dumping of mine waste. The primary environmental risk associated with this waste is the leaching of toxic metals from dumps of spoil or refuse into the subsurface soil or into nearby water resources. The extent of metal release is controlled via the oxidative dissolution of pyrite and potential re-sequestration through secondary Fe oxides. The characterization of the dominant Fe-bearing phase and the distribution of trace metals associated with these phases was determined via electron microscopy, synchrotron-based X-ray micro-fluorescence (μ-XRF) element and redox mapping from shallow mine soils from an impacted watershed in Appalachian Ohio. The dominant Fe-bearing phases were: (1) unweathered to partially weathered pyrite; (2) pseudomorphic replacement of pyrite with Fe(III) oxides; (3) fine-grained Fe oxide surface coatings; and (4) discrete Fe(III) oxide grains. Thicker secondary coatings and larger particles were sulfate rich, whereas smaller grains and thinner coatings were sulfate poor. The discrete Fe oxide grains exhibited the highest concentrations of Cr, Mn, Ni, and Cu, and sub-grain-scale concentration trends (Mn &gt; Cr &gt; Ni &gt; Cu) were consistent with bulk soil properties. Predicting future metal transport requires an understanding of metal speciation and distribution from the sub-grain scale to the pedon scale.

Kinetic Investigation of Initial Oxidative Dissolution of Pyrite in Alkaline Media (pH 9–12) and Influence of Ca and Mg: A Fundamental Study for Pyrite Depression in Froth Flotation

Kinetic Investigation of Initial Oxidative Dissolution of Pyrite in Alkaline Media (pH 9–12) and Influence of Ca and Mg: A Fundamental Study for Pyrite Depression in Froth Flotation

The role of Acidithiobacillus ferrooxidans in Fe(II) oxidation of pyrite in bioleaching processes

AbstractBACKGROUNDMicrobial leaching is an emerging ore extraction technology, especially for low‐grade ores. In sulfide and oxide ore bioleaching systems, Acidithiobacillus ferrooxidans is becoming a widely applied microorganism for mineral processing. Many studies are currently focusing on the vital role of A. ferrooxidans in facilitating the efficiencies of sulfur and iron species transformation for sulfide ore bioleaching. Existing research usually investigates the composite action of A. ferrooxidans and Fe(III) for ore bioleaching, the effect of A. ferrooxidans alone being investigated to a lesser extent. An important effect is the role of A. ferrooxidans in the oxidative conversion of Fe(II) in pyrite bioleaching.RESULTSThe oxidative dissolution of pyrite in an imitated A. ferrooxidansbioleaching system was investigated in this work, particularly concerning the effects of bacteria on Fe(II) oxidation of the pyrite. Results of the dissolution performance test, cyclic voltammetry and polarization analysis indicated that the preceding oxidation of Fe(II) has control over the pyrite dissolution rate in the imitated bioleaching system. The semiconductor properties and carrier density of the passivation films of pyrite formed under a variety of conditions indicated that the oxidative leaching of ferrous disulfide is subject to the movement of the holes of the Fe2+species and the oxygen vacancy of H2O at the interface. Acidithiobacillus ferrooxidans promotes pyrite bioleaching by accelerating the preferential oxidation of Fe(II) caused by movement of the electron hole of Fe2+ to H2O at the solid–liquid interface.COCLUSIONThe oxidation process of pyrite in this study turns out to involve hole movement. © 2022 Society of Chemical Industry (SCI).

Geochemical element mobilisation by interaction of Bowland shale with acidic fluids

Hydraulic fracturing is widely used to exploit unconventional hydrocarbon sources, to enhance exploitation of geothermal energy and to aid in carbon sequestration through underground storage of captured CO2. The hydraulic fracturing fluids, which are commonly acidic, cause dissolution of minerals and desorption of elements which can lead to groundwater contamination. Batch reactor experiments were conducted to explore the interaction of simulated fracturing fluids with two end member compositions of basinal shales of the Bowland-Hodder unit (Carboniferous, UK) whereby the impact of temperature, fluid acidity, and rock/fluid ratio conditions were investigated. The results demonstrate that the fluid acidity is mainly controlled by the oxidative dissolution of pyrite and the dissolution of calcite, impacting mobilisation and fate of major and trace elements. The dissolution of calcite and pyrite significantly dominates the leaching of Sr and As, respectively. Generally, increased fluid acidity and temperature facilitate element mobilisation due to enhanced mineral dissolution and ion desorption, whereas higher rock/fluid ratio (higher mass of carbonate minerals) raises the buffering capacity and may promote the immobilisation of some metal ions by adsorption and precipitation (e.g. Ba, Pb, Fe, Al, and Mn). Moreover, the surface topography of different minerals in polished shale sample sections after fluid-rock interaction indicates that mineralogical compositions may play an important role in determining the pore structure. This research identifies chemical reaction pathways of geochemical elements (including contaminants) in fracturing fluids over a range of fluid chemistries and environmental conditions, and helps to evaluate element mobilisation from shale reservoirs with differing mineralogies.

Effect of salinity on the kinetics of pyrite dissolution in oxygenated fluids at 60 °C and implications for hydraulic fracturing

Shale hydraulic fracturing involves the injection of oxygenated acidic fluids which trigger redox driven and acid driven reactions, in particular with pyrite and carbonates. These reactions alter the shale pore and fracture network and mobilize potentially harmful elements. Hence, understanding the kinetics of pyrite oxidation at shale reservoir conditions is critical. Here, we investigate the effect of salinity on the rate of pyrite oxidative dissolution by fluids of pH 1 at 60 °C, and 0–3.4 mol L−1 NaCl. We determine a pyrite dissolution rate of about 1.2 × 10−9 mol m−2 s−1 in non-saline oxygenated acidic fluids at 60 °C, consistent with previously established rate equations, considering an activation energy of 57 kJ mol−1. Our results suggest the following correlation between the pyrite dissolution rate r (in mol m−2 s−1) at 60 °C and Eh (0.580–0.700 V), log(r) = 4.47 Eh – 12.03. This reaction order is lower than that established in previous studies involving pyrite reaction with Fe3+ enriched fluids in the presence of dissolved O2. Furthermore, we calculate that the pyrite oxidation rate varies from 0.8 × 10−10 to 1.3 × 10−10 mol m−2 s−1 at 25 °C, based on salinity-determined dissolved oxygen content. The mixed flow reactor experimental results suggest a dependence of pyrite oxidation rate on Cl− activity aCl (in mol kg−1) at 60 °C for fluids with 0.6–3.4 mol L−1 NaCl and Eh of 612 ± 8 mV as follows, log(r) = −0.58 log(aCl) – 9.42. The pyrite oxidation rate varies within less than half an order of magnitude for the salinity range tested, and hence, the impact of salinity is less important than the Eh and temperature. The improved understanding of factors controlling pyrite oxidation kinetics may help in predicting environmental and engineering impacts of injection of hydraulic fracturing fluids in shale.

The Combined Effects of Galvanic Interaction and Silicate Addition on the Oxidative Dissolution of Pyrite: Implications for Acid and Metalliferous Drainage Control.

The aim of this study was to determine the combined effect of galvanic interaction and silicate addition on the dissolution of pyrite, the major contributor to acid and metalliferous drainage (AMD). Single (pyrite, sphalerite, and galena)- and bi-sulfide (pyrite-sphalerite and pyrite-galena) batch dissolution experiments were carried out with addition of 0.8 mM dissolved silicate for comparison to previously published data. The pyrite dissolution rate was reduced by 98% upon silicate addition at pH 7.4 with little effect at pH 3.0 and 5.0. The effect of galvanic interaction on reducing pyrite dissolution decreased with increasing pH and was greater in the presence of sphalerite than galena. In contrast, the effect of silicate addition increased with increasing pH and was greater in the presence of galena than sphalerite. The greatest combined effect was at pH 7.4, with <0.1% of pyrite leached in both bi-sulfide systems. Silicate addition also significantly reduced the dissolution of sphalerite or galena (by 10-44%, except at pH 3 for the pyrite-sphalerite system). These results suggest that silicate addition, for reducing both pyrite dissolution and metalliferous drainage, may be applicable at a broad pH in mixed sulfide systems.

Reactive fronts in chemically heterogeneous porous media: Experimental and modeling investigation of pyrite oxidation

The spatial distribution of reactive minerals in subsurface porous media is an important control for groundwater quality. In this study we investigate pyrite oxidation reactive fronts in chemically heterogeneous porous media by combining laboratory experiments and reactive transport modeling. We performed experiments in different setups including batch, 1-D column, and 2-D flow-through systems. The flow-through experiments were performed in physically homogenous but chemically heterogeneous domains with embedded reactive pyrite inclusions at different spatial locations and with different concentrations. The setups were initially maintained under anoxic conditions and subsequently flushed with an inflowing oxic solution. A non-invasive optode technique was used for high-resolution monitoring of oxygen. This allowed us to capture the dynamics of the reactive oxygen fronts in the 1-D columns and in the 2-D flow-through chamber. Water quality analyses of the products of pyrite oxidation, iron (Fe) and sulfur (S), were also carried out in the different setups. The concentration of these species released in the 1-D columns (up to 6.2 × 10−5 mol/L Fe and 13.7 × 10−5 mol/L S) and in the 2-D setup (up to 3 × 10−6 mol/L Fe and 9 × 10−6 mol/L S) could be quantitatively related to the consumption of oxygen (up to 1.9 × 10−4 mol/L consumed in the 1-D and 2-D setups). The reaction rates were found to be different between the setups and dependent on the spatial location and concentration of the pyrite inclusions.A modeling approach coupling 1-D and 2-D transport codes with the geochemical simulator PHREEQC is proposed to simulate the spatial and temporal dynamics of oxygen transport, the kinetics of pyrite oxidative dissolution, and the changes in water quality in the chemically heterogeneous flow-through setups. The model allowed the quantitative interpretation of the experimental results and represents a valuable tool to capture the coupling between multidimensional transport and geochemical reactions both in laboratory and in field scale applications.

Chemical Forms of Mercury in Pyrite: Implications for Predicting Mercury Releases in Acid Mine Drainage Settings.

Pyrite (cubic FeS2) is the most abundant metal sulfide in nature and also the main host mineral of toxic mercury (Hg). Release of mercury in acid mine drainage resulting from the oxidative dissolution of pyrite in coal and ore and rock resulting from mining, processing, waste management, reclamation, and large construction activities is an ongoing environmental challenge. The fate of mercury depends on its chemical forms at the point source, which in turn depends on how it occurs in pyrite. Here, we show that pyrite in coal, sedimentary rocks, and hydrothermal ore deposits can host varying structural forms of Hg which can be identified with high energy-resolution XANES (HR-XANES) spectroscopy. Nominally divalent Hg is incorporated at the Fe site in pyrite from coal and at a marcasite-type Fe site in pyrite from sedimentary rocks. Distinction of the two Hg bonding environments offers a mean to detect microscopic marcasite inclusions (orthorhombic FeS2) in bulk pyrite. In epigenetic pyrite from Carlin-type Au deposit, up to 55 ± 6 at. % of the total Hg occurs as metacinnabar nanoparticles (β-HgSNP), with the remainder being substitutional at the Fe site. Pyritic mercury from Idrija-type Hg deposit (α-HgS ore) is partly divalent and substitutional and partly reduced into elemental form (liquid). Divalent mercury ions, mercury sulfide nanoparticles, and elemental mercury released by the oxidation of pyrite in acid mine drainage settings would have different environmental pathways. Our results could find important applications for designing control strategies of mercury released to land and water in mine-impacted watersheds.

The geochemical effects of O2 and SO2 as CO2 impurities on fluid-rock reactions in a CO2 storage reservoir

Costs for CO2 capture could be reduced if CO2 gas impurities can be co-injected and do not adversely affect the long-term CO2 containment. This project, as a part of the Callide Oxyfuel Project, investigates the geochemical impacts of the CO2 impurities SO2 and O2 on mineral-fluid reactions in a siliciclastic reservoir. In a single-well push-pull field experiment CO2-saturated water with and without impurities was injected into the reservoir. The injection water was allowed to interact with minerals in the reservoir for three weeks, during which water was back-produced and sampled on three occasions. Four soluble tracers were added to the injection water to estimate the proportions of injection and formation water in the back-produced water. Redox state, speciation and reaction pathway modelling are used as part of the data interpretation. Once injected, SO2 (67ppm vol/vol, initially as a dissolved impurity in CO2) was dissolved and oxidised, leading to sulphate formation. The alkalinity of the injection water counteracted any substantial decrease in pH, which would otherwise occur due to sulphuric acid formation, thus inhibiting additional mineral dissolution. After being injected, O2 (6150ppm vol/vol, dissolved impurity in CO2) led to immediate oxidative dissolution of pyrite. Consequently, the SO42− concentration increased rapidly and dissolved iron is predicted to precipitate as hematite. Overall, the impact of CO2 impurities was minimal.

Acid rock drainage in Nevado Pastoruri glacier area (Huascarán National Park, Perú): hydrochemical and mineralogical characterization and associated environmental implications.

The generation of acid rock drainage (ARD) was observed in an area of Nevado Pastoruri as a result of the oxidative dissolution of pyrite-rich lutites and sandstones. These ARDs are generated as abundant pyrite becomes exposed to atmospheric conditions as a result of glacier retreat. The proglacial zone contains lagoons, springs, streams and wetlands, scant vegetation, and intense fluvioglacial erosion. This work reports a comprehensive identification and the results of sampling of the lagoons and springs belonging to the microbasin, which is the headwaters of the Pachacoto River, as well as mapping results based on the hydrochemical data obtained in our study. The physical properties and water chemistry of 12 springs and 22 lagoons from the proglacial zone are also presented. Water springs are far from being chemically uniform, with pH and EC values ranging between 2.55-6.42 and 23-1110μS/cm respectively, which suggests a strong geologic control on water chemistry. Fe-SO4-2 concentrations confirm the intense process of pyrite oxidative dissolution. Many of the lagoons are affected by ARD, with low pH (~3), and high EC (256-1092μS/cm) values when compared with unaffected lagoons (EC between 7 and 59μS/cm), indicating a high degree of mineralization. The affected lagoons show higher concentrations of SO42- and SiO2, and elements as Fe, Al, Mg, Mn, Zn, Co, and Ni, which are related to the alteration of pyrite and the dissolution of aluminosilicate minerals. Schwertmannite-goethite appears to be the most important mineral phases controlling the Fe solubility at a pH of 2-3.5. Moreover, they act as a sorbent of trace elements (As, Sb, V, Pb, Zn, Cr), which is an efficient mechanism of natural attenuation. Despite of this, the water flowing out from the basin is acid (pH3.1) and contains significant concentrations of Fe (0.98mg/L) and Al (3.76mg/L) that confer mineral acidity to water. The Pachacoto River located 5.5km downstream from this point showed a strong natural attenuation, with a pH of 6.9 and low concentration of metals. This mitigating process is possible due to (i) the formation of precipitates that retain toxic elements and (ii) the mixing with natural waters that promote dilution, which favor the increase of pH until circumneutral conditions.

Pyrite oxidation in air-equilibrated solutions: An electrochemical study

Oxidative dissolution of pyrite (FeS2) in air-equilibrated solutions was investigated with electrochemical techniques (polarization measurements and electrochemical impedance spectroscopy) at different solution pH (1 to 5) and temperatures (25 to 40°C). We have found that, at a given temperature, the oxidation current density (jox) varies only slightly from initial pH1 to 4, but was significantly lower at initial pH5, while oxidation potential (Eox) decreases when pH increases. At the investigated temperatures, the reaction order with respect to hydrogen ions has been found to be zero, suggesting that H+ is not involved into the rate-determining step of the oxidation rate. The jox values increase with temperature, whereas Eox potentials are hardly affected by temperature from 25 to 40°C. Activation energies (Ea) were estimated from polarization data and vary between 57±28kJ/mol (initial pH5) and 19±3kJ/mol (initial pH4). The obtained values indicate that the rate-determining step of the overall oxidative dissolution of FeS2 involves a surface reaction. The importance of surface reaction is also supported by the electrochemical impedance spectroscopy measurements which show that interfacial transfer of electrons is dominated by surface charge transfer and the elementary surface reaction which controls the mineral oxidative dissolution is the electron transfer from the conduction band of pyrite to adsorbed O2.

Pyrite oxidation in the presence of hematite and alumina: II. Effects on the cathodic and anodic half-cell reactions.

The oxidative dissolution of pyrite is an important process in the redox recycling of iron (Fe) and is well-known for its role in the formation of acid mine drainage (AMD), which is considered as the most serious and widespread problem after the closure of mines and mineral processing operations. Because this process requires the movement of electrons, common metal oxides in nature that have either semiconducting (e.g., hematite) or insulating (e.g., alumina) properties may have strong effects on it. In this study, changes in the electrochemical behavior of pyrite in the presence of hematite and alumina were investigated. Results showed that the formation of surface-bound species directly influenced the anodic and cathodic half-cell reactions as well as the transfer of electrons between these sites. Pyrite pretreated in the air became anodically more reactive than that pretreated in oxygenated water, but the type of oxidizing media had little effect on the cathodic half-cell reaction. The presence of hematite and alumina during pretreatment also had strong effects on the electrochemical properties of pyrite. Chronoamperometry measurements suggest that hematite and alumina enhanced the anodic half-cell reaction but suppressed the cathodic half-cell reaction of pyrite oxidation. Increased anodic half-cell reaction in the presence of hematite could be attributed to electron "bridging" and catalytic effects of this mineral. In contrast, the effects of alumina on the anodic half-cell reaction were indirect and could be explained by the formation of Fe3+-oxyhydroxide surface species during pretreatment. Suppression of the cathodic half-cell reaction by both minerals was attributed to their "protective" effect on cathodic sites. Our results also point to the cathodic half-cell reaction as the rate determining-step of the overall oxidative dissolution process.

Computational Efficiency of Decoupling Approach in Solving Reactive Transport Model: A Case Study of Pyrite Oxidative Dissolution

Pyrite existed widely in nature and its oxidative dissolution might lead groundwater to become acidic, which was harmful to the environment and indeed to artificial building materials. The reactive transport model was a useful tool to predict the extent of such pollution. However, the chemical species were coupled together in the form of a reaction term, which might lead the equations to be nonlinear and thus difficult to solve. A decoupling approach was presented: linear algebraic manipulations of the stoichiometric coefficients of the chemical reactions for the purpose of reducing the number of equation variables and simplifying the reactive source were used. Then the original and decoupled models were solved separately, by both a direct solver and an iterative solver. By comparing the solution times of two models, it was shown that the decoupling approach could enhance the computational efficiency, especially in situations using denser meshes. Using a direct solver, more solution time was saved than when using an iterative version.

Mobilization of metals from Eau Claire siltstone and the impact of oxygen under geological carbon dioxide sequestration conditions

To investigate the impact of O2 as an impurity co-injected with CO2 on geochemical interactions, especially trace metal mobilization from a geological CO2 sequestration (GCS) reservoir rock, batch studies were conducted with Eau Claire siltstone collected from CO2 sequestration sites. The rock was reacted with synthetic brines in contact with either 100% CO2 or a mixture of 95 mol% CO2-5 mol% O2 at 10.1MPa and 75°C. Both microscopic and spectroscopic measurements, including 57Fe-Mössbauer spectroscopy, Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry, powder X-ray diffraction, scanning electron microscopy-energy dispersive X-ray spectroscopy, and chemical extraction were combined in this study to investigate reaction mechanisms. The Eau Claire siltstone contains quartz (52wt%), fluorapatite (40%), and aluminosilicate (5%) as major components, and dolomite (2%), pyrite (1%), and small-particle-/poorly-crystalline Fe-oxides as minor components. With the introduction of CO2 into the reaction vessel containing rock and brine, the leaching of small amounts of fluorapatite, aluminosilicate, and dolomite occurred. Trace metals of environmental concern, including Pb, As, Cd, and Cu were detected in the leachate with concentrations up to 400ppb in the CO2–brine–rock reaction system within 30days. In the presence of O2, the oxidation of Fe(II) and the consequent changes in the reaction system, including a reduction in pH, significantly enhanced the mobilization of Pb, Cd, and Cu, whereas As concentrations decreased, compared with the reaction system without O2. The presence of O2 resulted in the formation of secondary Fe-oxides which appear to be Fe(II)-substituted P-containing ferrihydrite. Although the rock contained only 1.04wt% total Fe, oxidative dissolution of pyrite, leaching and oxidation of structural Fe(II) in fluorapatite, and precipitation of Fe-oxides significantly decreased the pH in brine with O2 (pH 3.3–3.7), compared with the reaction system without O2 (pH 4.2–4.4). In the CO2–rock–brine system without O2, the majority of As remained in the rock, with about 1.1% of the total As being released from intrinsic Fe-oxides to the aqueous phase. The release behavior of As to solution was consistent with competitive adsorption between phosphate/fluoride and As on Fe-oxide surfaces. In the presence of O2 the mobility of As was reduced due to enhanced adsorption onto both intrinsic and secondary Fe-oxide surfaces. When O2 was present, the dominant species in solution was the less toxic As(V). This work will advance our understanding of the geochemical reaction mechanisms that occur under GCS conditions and help to evaluate the risks associated with geological CO2 sequestration.

Biogeochemical processes governing natural pyrite oxidation and release of acid metalliferous drainage.

The oxidative dissolution of sulfide minerals (principally pyrite) is responsible for the majority of acid metalliferous drainage from mine sites, which represents a significant environmental problem worldwide. Understanding the complex biogeochemical processes governing natural pyrite oxidation is critical not only for solving this problem but also for understanding the industrial bioleaching of sulfide minerals. To this end, we conducted a simulated experiment of natural pyrite oxidative dissolution. Pyrosequencing analysis of the microbial community revealed a distinct succession across three stages. At the early stage, a newly proposed genus, Tumebacillus (which can use sodium thiosulfate and sulfite as the sole electron donors), dominated the microbial community. At the midstage, Alicyclobacillus (the fifth most abundant genus at the early stage) became the most dominant genus, whereas Tumebacillus was still ranked as the second most abundant. At the final stage, the microbial community was dominated by Ferroplasma (the tenth most abundant genus at the early stage). Our geochemical and mineralogical analyses indicated that exchangeable heavy metals increased as the oxidation progressed and that some secondary sulfate minerals (including jarosite and magnesiocopiapite) were formed at the final stage of the oxidation sequence. Additionally, we propose a comprehensive model of biogeochemical processes governing the oxidation of sulfide minerals.

Electrochemical Investigation of the Mechanism of Aqueous Oxidation of Pyrite by Oxygen

The aqueous oxidation of pyrite in air-equilibrated HCl solutions was investigated at various temperatures (30 and 40 oC), pH values (2 to 5) and sulfate concentrations (0.001 to 0.5M). It was found that the oxidation current densities (iox) increase when the temperature and sulfate concentration increase. The rate dependence on sulfate concentration indicates that during the oxidative dissolution of pyrite an activated complex of ionic nature is formed on mineral surface, and sulfate is not involved in the rate determining step of pyrite oxidative dissolution.

Modeling of oxygen gas diffusion and consumption during the oxic transient in a disposal cell of radioactive waste

The oxic transient in geological radioactive waste disposals is a key issue for the performance of metallic components that may undergo high corrosion rates under such conditions. A previous study carried out in situ in the argillite formation of Tournemire (France) has suggested that oxic conditions could have lasted several years. In this study, a multiphase reactive transport model is performed with the code HYTEC to analyze the balance between the kinetics of pyrite oxidative dissolution, the kinetics of carbon steel corrosion and oxygen gas diffusion when carbon steel components are emplaced in the geological medium. Two cases were modeled: firstly, the observations made in situ have been reproduced, and the model established was then applied to a disposal cell for high-level waste (HLW) in an argillaceous formation, taking into account carbon steel components and excavated damaged zones (EDZ).In a closed system, modeling leads to a complete and fast consumption of oxygen in both cases. Modeling results are more consistent with the in situ test while considering residual voids between materials and/or a water unsaturated state allowing for oxygen gas diffusion (open conditions). Under similar open conditions and considering ventilation of the handling drifts, a redox contrast occurs between reducing conditions at the back of the disposal cell (with anoxic corrosion of steel and H2 production) and oxidizing conditions at the front of the cell (with oxic corrosion of steel). The extent of the oxidizing/reducing front in the disposal cell is strongly dependent on the gas diffusion coefficient in partially saturated zones.

Oxidative dissolution of pyrite in acidic media

The pyrite oxidative dissolution in air-saturated (AS), H2O2, and Fe3+ solutions at pH 2.5 and 25 °C was investigated by electrochemical and aqueous batch experiments. The corrosion current density (icorr) increases from AS solution to Fe3+ and H2O2 solutions. For the same oxidant, icorr increases when the concentration of the oxidant increases. Similar variation was observed for the corrosion potential (Ecorr). Electrochemical impedance spectroscopy measurements have indicated that in AS and H2O2 solutions, the charge transfer is the rate determining step of pyrite oxidative dissolution. In the presence of Fe(aq)3+, both the charge transfer process and mass transfer caused by the diffusion of oxidant or reaction products across the interface of electrode control the mineral oxidative dissolution. The corrosion current densities of oxidative dissolution measured by electrochemical methods are higher than those estimated from dissolution rates determined by aqueous bath experiments. The observed differences suggest that the mechanism of polarized electrode oxidation is different by the mechanism of pyrite oxidation under open circuit conditions.